Defect-Induced Efficient Partial Oxidation of Methane over Nonstoichiometric Ni/CeO₂ Nanocrystals

We report the development of a highly crystalline Ni/CeO₂ catalyst with varying amounts of Ni content using ammonium carbonate complex solution of cerium­(IV) at a low temperature. The catalyst was characterized by XRD, XPS, BET-surface area, TEM, and H₂-TPR analysis. We have observed that the maximum incorporation of Ni in CeO₂ crystal system as the substitution point defect took place up to impregnation of 2.5% Ni, and an almost maximum reduction of unit cell parameter was observed. Further increase in the amount of Ni, the additional Ni may create interstitial point defects with surface defects. The synthesized catalysts showed defect-dependent catalytic activity for low temperature (∼450 °C) methane activation to form synthesis gas. The 7.5 wt % Ni/CeO₂ catalyst showed 98% conversion of methane with 73 and 71% selectivity of CO and H₂, respectively, at 800 °C without any deactivation until 50 h on time on stream. We also believe that with enhancement of Ni loading, the interstitial point defects and the surface defects due to the formation of the Ce–O–Ni–O– layer, with under-coordinated oxygen atom, on the surface may be the possible reason for the high activity of the catalyst with Ni loading between 5 and 7.5 wt %. With further increase of Ni loading, the Ni nanoparticles were formed with the expense of the Ce–O–Ni–O– layer and in-turn decreased the catalytic activity